The present invention relates to switching circuits and more particularly, to a "H" switching circuit that is suitable to be used to drive a step motor or the like.
In its basic form, the "H" switching circuit generally is comprised of a pair of upper switching transistors and a pair of lower switching transistors. Each upper switching transistor is coupled to a respective output between which a load is to be connected such that the upper switching transistor is cross coupled therethrough to the opposite lower switching transistor thereby forming first and second paired switching transistor circuits.
In operation the first and second paired switching transistor circuits may be alternately rendered conductive to alternately reverse the polarity of current drive flowing through the load. The structure and operation of the aforedescribed "H" switch circuit is generally understood by those skilled in the art.
A problem can arise in the above described "H" switching circuit due to the parasitic collector-base capacitances of the two upper switching transistors in conjunction with the parasitic capacitances of any driving transistors that may be connected therewith. For example, current spikes can be generated at the base of the respective upper switching transistor that is being turned off in response to the other paired switching transistor circuit being turned on. This parasitic current spike is amplified by the particular upper transistor and is passed directly to the lower transistor of the other one of the paired switching transistor circuits which is directly connected therewith. Not only is the amount of load current undesirably reduced by this parasitic capacitance produced large current spike, but there also arises undesirable die heating, circuit metal migration and attendant reliability problems.
Therefore, a need exists for a "H" switching circuit having improved switching characteristics in which parasitic capacitance produced current spikes are severely limited.